How to Fake a UFO Landing

A flying saucer descends onto an open field and lands, kicking up dust
all around it. If this happened in a remake of The Day the Earth Stood
Still nobody would blink. But imagine that instead of a big beautiful
image executed with the precision and care of a big-budget feature,
what you're watching looks like it fell on the cutting-room floor of
The Blair Witch Project. You're not seeing this in a comfortable
screening room at your local cinema, where the picture is clean, sharp
and bigger than life, but rather you're standing gathered around
a booth at your local science-fiction convention. The guy playing the
video isn't a producer. He isn't even an independent filmmaker. He's a guy
who's genuinely convinced that this video can't be faked. If it were,
he says, the seams would show, and whoever gave this to him really did
record evidence of alien visitation that the government is covering up,
and that by showing this publicly, he's taking a terrible risk. But, he
feels he must expose the fraud that governments and aliens perpetrate
on unsuspecting citizens!

This scenario may sound like it clawed its way out of the
X-Files'
wastebasket, but as VisualFX technology gets ever cheaper and more
ubiquitous, faking a video like this becomes no problem. Of course,
it takes a lot of expertise and dedication to get the colors, shadows
and reflections to match convincingly. One would think that getting the
movement to match as the camera person runs and zooms with a handheld
shot would be the most difficult part of the equation. Once upon a time,
this was true.

It used to be that the only way you could achieve the movement precision
necessary to sell an effect like this was to put your camera on a motion
control rig and have a computer record the movements in the field and
then reproduce them exactly (though to a smaller scale) in the post house
where the artificial elements (in our case, the flying saucer and the dust)
were photographed. Aside from being very cumbersome and expensive, this
approach sharply limited the kinds of shots that an FX artist could do
to those that could be reproduced by an electric gimbal and a prime lens.

No longer. The late 1990s saw a great flowering of research and development
into the area of computerized match moving—matching the movement of
different visual elements so that they appeared to exist organically
in the same scene. Putting the computer in the mix both at the match
moving and at the compositing stage gives a lot more control and freedom
than previously.

Why Use Match-Moving Software at All?

Human visual acuity isn't the best on the planet, but it is startlingly
good. With a little practice, an ordinary fellow sitting in the
audience for The Matrix can spot the grain mismatch in shots that were
too hastily done. Our visual cortex does the differential calculus to
tell us “this doesn't belong here”. It follows that this same mental
apparatus could be employed to create the trickery in the first place.
With most VisualFX work, there are complicated tools, and then there is
doing it by hand. Like most other fields of human endeavor, the better
an artist is, the fewer training wheels he or she generally will rely on.
So, why not do match moving by hand?

The short answer is that many artists do, under some circumstances.
Other times, there is an interaction between the artist and the match-moving software, with the artist choosing points for the software to
track, either because the tool doesn't detect the right points, latches on
to points that aren't appropriate or doesn't do point detection at all.

However, the art of motion tracking is nontrivial. Although our visual
cortexes are excellent at detecting error, they are somewhat less
excellent at projecting perfection outward. We do not create
grand, realistic paintings naturally—indeed, we have to be taught to see light,
shadow, form and so on in a certain way in order ponder even attempting to
work like a Bouguereau or a Leonardo. Similarly, our ability actually
to distinguish motion that doesn't fit is quite keen, although our ability to
create a perfect motion path is coarse by comparison—something we
don't notice until after we play it back and see the drift creeping in
even with the most careful hand-tracks.

Of course, a match-moving program won't always get a perfect track,
but the interaction of a good artist with a good program delivers
top-notch results.

Why Get a Voodoo Doll?

Aside from the fact that it's free, why use Voodoo for this project?

The truth is that Voodoo isn't going to solve every match-moving problem,
even leaving out the ultra-delicate moves that the higher-end match movers
handle better. The field of match moving is basically divided in two:
2-D motion tracking and 3-D camera tracking.

2-D motion tracking is the technology used in compositors to affix a
new element to a specific point in the frame. A user generally
will select one or two feature points, and the computer then will follow
the points around the frame as the objects move within it. When the
tracker slides off the selected point, the artist gently will correct
it to keep the track from drifting. Two commonplace examples of this
process can be seen in blurring suspects' faces on
Cops! and in placing
virtual advertisements on infield walls at baseball games. 2-D tracking
tracks only the position of an object within the frame, which gives it
a double-barreled Achilles' heel: parallax and perspective.

Parallax is the phenomenon whereby foreground objects seem to move
faster than do background objects. As your point of view moves, the
angle at which you perceive objects changes subtly, which is why you see
a parallax driving down the road. With 2-D tracking, your track marks
are pretty much all you get. This can be a problem if, for instance,
you're moving over a greenscreen and the digital set is supposed to
extend for quite a ways down in depth. As soon as you add depth to
lateral movement, particularly when your track marks are close to the
camera, you need to work in 3-D, or you have to fake parallax by
hand—a dubious and difficult undertaking that easily shatters the illusion
you're trying to create. A really good artist can pull it off, but it
takes a lot of practice.

Perspective is the other wild card in the equation. Lenses do not see
the world as it is. Instead, every lens distorts the world in certain
mathematically predictable ways. This distortion is closely related to
focal length and aperture, and measuring the distortion accurately is
essential to tracking elements properly in the shot. This wild card
gets even wilder with zoom (extending the lens to get a closer shot)
and dolly-in (moving the camera toward a subject) movements, which
involve constantly changing perspective in one fashion or another along
the z axis, which is the axis that 2-D motion tracking can't cope with.
Perspective changes also can be faked, but it's far more difficult than
faking parallax and far more time consuming.

This is where 3-D camera tracking comes in. Instead of simply tracking the
location of certain user-selected features to create a good 2-D track, the
computer attempts to guess the position and motion of the camera based
on the footage. Pitch, yaw, roll and lens length are all calculated
based solely on the finished video (though any information you have and
can input manually will make it work faster). The ability to
reconstruct all these parameters accurately means that the problems of parallax and
perspective are solved, even during dolly and zoom moves. Needless to
say, this is a mathematically complex process designed to test the minds
of even the most ardent effects artist who wasn't also a comp-sci or
optics major at a university. Nonetheless, the algorithms for pulling
this off are well known and included in most camera-tracking packages.

Although most 2-D motion trackers are built in to existing compositing systems
(such as After Effects), 3-D camera trackers operate on a standalone
basis and export their data—camera settings and movement, as well
as the “point cloud”—to various 3-D programs, and it is
in the 3-D
program where the magic happens. The 3-D program also gives an extra
measure of control and refinement beyond what the tracker itself allows,
as you can tweak the camera animation curves.

I said earlier that the 1990s saw a lot of funding into creating software
like this. Well, as every tech-junkie knows, where thy research
funding is, there thy grad students also will be. Thanks to a team of
particularly dedicated grad students in Hannover, Germany, the technology
to match camera movement in three dimensions is available to Linux and
Windows users for free—a very good deal, considering that comparable
commercial packages run upward of several thousand dollars a seat.
For the savings, you do sacrifice some sophistication in the ability to
fine-tune your shot, but for most applications, Voodoo does very
well.

So, grab a copy of it, and let's get you ready for your appearance on
the Art Bell show, peddling your newest Genuine UFO Video (tm)!

Trending Topics

Upcoming Webinar

Getting Started with DevOps - Including New Data on IT Performance from Puppet Labs 2015 State of DevOps Report

August 27, 2015
12:00 PM CDT

DevOps represents a profound change from the way most IT departments have traditionally worked: from siloed teams and high-anxiety releases to everyone collaborating on uneventful and more frequent releases of higher-quality code. It doesn't matter how large or small an organization is, or even whether it's historically slow moving or risk averse — there are ways to adopt DevOps sanely, and get measurable results in just weeks.